In order to individualize training intensity according to cardiovascular and metabolic stress (exercise stress), rather than absolute external workload, different methods for determining exercise intensity have been used. These methods have been, for example, based on % HRmax, % VO2max, % HRreserve (% HRR) or % VO2reserve (% VO2R). In addition, intensity zones that are based on metabolic thresholds have been used. Lactate threshold (LT) and Anaerobic threshold (AnT) (or onset of blood lactate accumulation OBLA) are such metabolic thresholds corresponding to a lactate threshold of about 2.5 mmol/l and 4.0 mmol/l, respectively. Training zones have been divided between these thresholds: 1) “Basic endurance training” or “long slow distance” below LT, including all steady pace exercises in which lactate is below 2.0 or 2.5 mmol/l level; 2) “Threshold training” between LT and AnT including steady pace and interval training with lactate values between 2.5 and 4.0 mmol/1; 3) VO2max training above AnT with lactate values over 4.0 mmol/l. As would be understood by a person of ordinary skill in the art, other intensity zone models have been applied in training. They may be informative in training practice, but it is important to note that they are not based on clearly defined physiological markers.
AnT may be metabolically characterized as the highest workload at which the body is able to achieve steady-state condition, which means that the lactate (specifically lactic acid) accumulation and removal (by metabolizing) is in balance so that lactate level stays stable. For example, if a person's AnT pace for running is 4:00 min/km, the person is able to run with that speed with a constant lactate level of about 4 mmol/l for prolonged periods. If the person increases pace to, for example 3:50 min/km, he/she may no longer able to achieve a steady state. Instead the person's lactate level may accumulate from an approximate 4 mmol/l starting level up to 10 mmol/l or higher until subjective fatigue takes place. Further, AnT may not be an exact lactate level but may vary between individuals. Lactate level corresponding to AnT may usually be between 3.0 and 4.0 mmol/l and may depend, for example, on personal physiological characteristics, or other factors as would be understood by a person of ordinary skill in the art.
Despite small individual variation in lactate levels corresponding to AnT, the same or similar physiological reactions may be related to it. For example, when exercise intensity is increased gradually from rest, at certain points anaerobic energy pathways usually start to noticeably activate and support the aerobic energy system in producing energy, which may sustain the energy demands of the body in the form of ATP. When anaerobic energy pathways are activated, glycogen/glucose can be used more rapidly to form ATP through glycolysis. This may result in fast lactate formation in the muscles. Until AnT, the lactate can be metabolized by the body without continuous accumulation. If exercise intensity is increased above AnT, aerobic energy production capabilities of the working muscles may have difficulties in matching the exercise energy requirements, and anaerobic energy production may increase rapidly. Consequently, lactic acid (lactate) may start to accumulate into the muscles and blood stream. When exercise intensity exceeds AnT, accumulation of lactic acid in muscles may cause fatigue in a brief period of time.
As would be understood by a person of ordinary skill in the art, similar terms may refer to the same physiological phenomenon as AnT. Non-limiting examples may include onset of blood lactate accumulation (OBLA), maximal lactate steady-state (MLSS), and respiratory compensation thresholds. All of these may refer practically to the same exercise intensity where lactic acid starts to accumulate due to the body's inability to remove lactic acid by oxidation and glucose re-formation (gluconeogenesis). This may cause a reduction in blood bicarbonate levels because bicarbonate can buffer the rise in acidity. Consequently, the body's carbon dioxide (CO2) production may be increased, thus possibly leading to increased CO2 removal from the body by means of increased respiration rate and ventilation. This rapid increase in ventilatory parameters can be used to detect these thresholds when exercise intensity is increased incrementally (for example, in test situations). Another detectable sign in incremental exercise tests may be the deflection point of heart rate. That is, at low to moderate intensities heart rate may increase linearly in relation to external work performed (e.g. speed or watts), but at AnT intensity, the increase in heart rate may start to slow. To clarify, this kind of metabolic threshold can be determined for everyone, whether one is sedentary, highly trained, or otherwise, but the exact lactate level may vary depending on individual physiological characteristics, for example between 3-4 mmol/l. Regardless of the exact lactate value, the same or similar physiologic responses may occur and these responses are measured in order to determine the threshold intensity. Heart rate level and/or pace (e.g. min/km or km/h) corresponding to AnT may be relevant training parameters since either one or both can be measured during any exercise and the user can easily observe whether he/she is on the right intensity zone or not. Pace (e.g. min/km or km/h) corresponding to AnT may allow an individual to, for example, track changes in fitness level because AnT-pace is a relevant predictor of, for example, marathon performance. Alternatively, other training parameters can be measured as would be understood by a person of ordinary skill in the art.
Accordingly, AnT may present an exercise intensity level that is important to long-term performances as it may represent the highest intensity of performance that can be tolerated for relatively long periods. The metabolic characteristics of AnT (and other similar lactate derived threshold values such as OBLA and MLSS) may be related to those of critical power, which is a concept that aims to represent the highest workload at which it is possible to perform, for example, 30 min to 60 min all-out time trials. In practice, the intensity may be higher at critical power than at AnT, and the AnT has been associated with a lower workload and increased time to exhaustion when compared to critical power.
Downsides of the current methodologies to define AnT are well known. The available methods for estimating the AnT require specific exercise protocols with incremental exercise intensity. Moreover, laboratory tests are invasive since blood lactate samples are used to determine AnT. Further, a single incremental test might not be accurate in each case, as there is variation in the individuals' performance level from day to day. Additionally, laboratory tests can cause anticipation and be stressful, which may affect physiology and the accuracy of the results. Therefore, it would be very beneficial if anaerobic threshold could be non-invasively analyzed from day to day with freely performed real-life exercises outside of laboratory conditions. This may be easier for the users, and as more data on determined anaerobic threshold may become available, it may also increase accuracy and reliability of the determined anaerobic threshold value.